Primary (non-rechargeable) electrochemical cells having an anode of lithium are known and are in commercial use. The anode is comprised essentially of lithium metal. Such cells typically have a cathode comprising manganese dioxide, and electrolyte comprising a lithium salt such as lithium trifluoromethane sulfonate (LiCF3SO3) dissolved in an organic solvent. The cells are referenced in the art as primary lithium cells (primary Li/MnO2 cells) and are generally not intended to be rechargeable. Alternative, primary lithium cells with lithium metal anodes but having different cathodes, are also known. Such cells, for example, have cathodes comprising iron disulfide (FeS2) and are designated Li/FeS2 cells. The iron disulfide (FeS2) is also known as pyrite. The Li/MnO2 cells or Li/FeS2 cells are typically in the form of cylindrical cells, typically AA size or AAA size cells, but may be in other size cylindrical cells. The Li/MnO2 cells have a voltage of about 3.0 volts which is twice that of conventional Zn/MnO2 alkaline cells and also have higher energy density (watt-hrs per cm3 of cell volume) than that of alkaline cells. The Li/FeS2 cells have a voltage (fresh) of between about 1.2 and 1.8 volts which is about the same as a conventional Zn/MnO2 alkaline cell. However, the energy density (watt-hrs per cm3 of cell volume) of the Li/FeS2 cell is higher than a comparable size Zn/MnO2 alkaline cell. The theoretical specific capacity of lithium metal is high at 3861.4 mAmp-hr/gram and the theoretical specific capacity of FeS2 is 893.6 mAmp-hr/gram. The FeS2 theoretical capacity is based on a 4 electron transfer from 4Li per FeS2 molecule to result in reaction product of elemental iron Fe and 2Li2S. That is, 2 of the 4 electrons change the oxidation state of +2 for Fe+2 in FeS2 to 0 in elemental iron (Fe0) and the remaining 2 electrons change the oxidation state of sulfur from −1 in FeS2 to −2 in Li2S.
Overall the Li/FeS2 cell is much more powerful than the same size Zn/MnO2 alkaline cell. That is for a given continuous current drain, particularly at higher current drain over 200 milliamp for an AA size cell, the voltage is flatter for longer periods for the Li/FeS2 cell than the Zn/MnO2 alkaline cell as may be evident in a voltage vs. time discharge profile. This results in a higher energy output obtainable from a Li/FeS2 cell compared to that obtainable for a same size alkaline cell. The higher energy output of the Li/FeS2 cell is more clearly and more directly shown in graphical plots of energy (Watt-hrs) versus continuous discharge at constant power (Watts) wherein fresh cells are discharged to completion at fixed continuous power outputs ranging from as little as 0.01 Watt to 5 Watt. In such tests the power drain is maintained at a constant continuous power output selected between 0.01 Watt and 5 Watt. (As the cell's voltage drops during discharge the load resistance is gradually decreased raising the current drain to maintain a fixed constant power output.) The graphical plot Energy (Watt-Hrs) versus Power Output (Watt) for the Li/FeS2 cell is above that for the same size alkaline cell. This is despite that the starting voltage of both cells (fresh) is about the same, namely, between about 1.2 and 1.8 volt.
Thus, the Li/FeS2 cell has the advantage over same size alkaline cells, for example, AAA, AA, C or D size or any other size cell in that the Li/FeS2 cell may be used interchangeably with the conventional Zn/MnO2 alkaline cell and will have greater service life, particularly for higher power demands. Similarly the Li/FeS2 cell which is a primary (nonrechargeable) cell can also be used as a replacement for the same size rechargeable nickel metal hydride cell, which has about the same voltage (fresh) as the Li/FeS2 cell. Thus, the primary Li/FeS2 cell can be used to power digital cameras, which require operation at high pulsed power demands.
The cathode material for the Li/FeS2 cell may be initially prepared in a form such as a slurry mixture (cathode slurry), which can be readily coated onto the metal substrate by conventional coating methods. The electrolyte added to the cell must be a suitable organic electrolyte for the Li/FeS2 system allowing the necessary electrochemical reactions to occur efficiently over the range of high power output desired. The electrolyte must exhibit good ionic conductivity and also be sufficiently stable, that is non reactive, with the undischarged electrode materials (anode and cathode components) and also non-reactive with the discharge products. This is because undesirable oxidation/reduction reactions between the electrolyte and electrode materials (either discharged or undischarged) could thereby gradually contaminate the electrolyte and reduce its effectiveness or result in excessive gassing. This in turn can result in a catastrophic cell failure. Thus, the electrolyte used in Li/FeS2 cell in addition to promoting the necessary electrochemical reactions, should also be stable to discharged and undischarged electrode materials. Additionally, the electrolyte should enable good ionic mobility and transport of the lithium ion (Li+) from anode to cathode so that it can engage in the necessary reduction reaction resulting in LiS2 product in the cathode.
An electrode composite is formed with a sheet of lithium, a sheet of cathode composite containing the FeS2 active material and separator therebetween. The electrode composite may be spirally wound and inserted into the cell casing, for examples, as shown in U.S. Pat. No. 4,707,421. A cathode coating mixture for the Li/FeS2 cell is described in U.S. Pat. No. 6,849,360. A portion of the anode sheet is typically electrically connected to the cell casing which forms the cell's negative terminal. The cell is closed with an end cap which is insulated from the casing. The cathode sheet can be electrically connected to the end cap which forms the cell's positive terminal. The casing is typically crimped over the peripheral edge of the end cap to seal the casing's open end. The cell may be fitted internally with a PTC (positive thermal coefficient) device or the like to shut down the cell in case the cell is exposed to abusive conditions such as short circuit discharge or overheating.
It would be desirable to improve the electrical conductivity of the cathode and to improve utilization (discharge efficiency of the cathode active material) in the Li/FeS2 cell. Consequently, it would be desirable to modify the FeS2 composition or its crystalline structure in order to produce a substitute, though related, material which has better electrical conductivity (lower resistivity) and demonstrates improved discharge characteristics compared to FeS2.
In C. H. Ho, C. E. Huang, and C. C. Wu, “Preparation and Characterization of Ni-Incorporated FeS2 Single Crystals”, Journal of Crystal Growth, Vol. 270 (2004), p. 535-541 is disclosed nickel (Ni)-doped FeS2 crystals. Nickel-incorporated single crystals with compositions Fe0.99S2:Ni0.01, Fe0.98S2:Ni0.02, Fe0.96S2:Ni0.04, and Fe0.9S2:Ni0.1 were grown by chemical vapor transport method (CVT) using ICl3 as a transport agent. By means of the analysis of the X-ray diffraction patterns, the series of nickel-doped FeS2 single crystals were determined to be single phase and isostructural. The conductivity measurements show the resistivity of the nickel-doped FeS2 decreased as the doping concentration of the nickel increased. There are no actual tests reported in this reference employing the nickel-doped FeS2 in batteries.
In A. Awano, K. Haraguchi, and H. Yamasaki, “Li/Fe1-xCoxS2 System Thermal Battery Performance”, Proceedings of the International Power Sources Symposium, 35th (1992), p. 219-222 is reported the evaluation of a synthetic iron-cobalt disulfide (Fe1-xCoxS2) as cathode material in a high temperature (thermal) battery. A preferred material was Fe1-xCoxS2 at x=0.15. (The atomic ratio of Fe to S is less than 0.5.) Such thermal batteries are rechargeable (secondary) batteries which operate at high temperature, e.g. at about 500° C., for example as in FeS2 thermal battery reported in U.S. Pat. No. 3,992,222. The synthetic Fe1-xCoxS2 cathode material was tested in a thermal battery having an anode which included lithium metal. The thermal battery was discharged at a test temperature of between about 450 to 600° C. It was concluded that the Fe1-xCoxS2 cathode exhibited improved discharge utilization and could be useful in primary thermal batteries, which characteristically operate at high temperature.
In Jae-Won Choi, et. al., “Effect of Metal Additives (Co and Ni) On the Electrochemical Properties of Lithium/FeS2 Batteries”, Materials Science Forum, Vols. 544-545 (2007), p. 973-976 there is reported an investigation of the rechargeable (cycleability) properties of Li/FeS2 cells with cathodes comprising FeS2, with and without low percent by weight cobalt (Co) or nickel (Ni). The cathode active material was prepared by high energy mechanical alloying technique from a starting mixture of iron, sulfur, and cobalt or nickel additive forming an alloy material. This mixture was subjected to ball milling performed in an argon atmosphere at ambient temperature with zirconium milling balls to produce an alloy of FeS2 and cobalt or nickel. It is not stated in this reference that the cobalt or nickel became incorporated into the crystalline structure of the FeS2. The discharge tests were made employing a rechargeable coin cell. It is stated that the room temperature cycleability of a Li/FeS2 cell had not shown good results for rechargeable batteries when the cathode active material was just FeS2. The test cells containing FeS2 and cobalt or FeS2 and nickel were subjected to charge/discharge cycles at room temperature as they were charged to 2.6V and discharged to 1.2V in each cycle for up to 5 cycles. The initial (first cycle) discharge capacity of a Li/FeS2 test cell with FeS2 cathodes containing 5 wt % cobalt or 3 wt % nickel were 571 mAmphr/g and 844 mAmphr/g respectively, compared to 389 mAmphr/g for the cathode with no metal additive. A principal objective was to try to improve the cycleability (rechargeable) characteristic of the Li/FeS2 cell by adding cobalt or nickel to the FeS2 cathode. The addition of cobalt or nickel to the FeS2 by ball milling was reported to improve the discharge capacity of the FeS2 cell attributed to the enhancement of electronic conductivity achieved by the addition of metallic material. The addition of cobalt as alloy to the FeS2 is reported to show results suitable for better rechargeable cycle performance. The cobalt or nickel was added to the FeS2 by ball milling (without heating at high temperature). There is no report or evidence presented to indicate whether the cobalt became incorporated into the FeS2 crystalline structure.
In Japanese patent publication Yamada JP57152673A a coin shaped lithium battery is disclosed having a lithium anode disc and a cathode active material comprising a solid solution formed of FeS2 and either copper (Cu) or zinc (Zn). The gram atom ratio of Cu/Fe may be 0.1/99.9 to 3.0/97.0 and the Zn/Fe gram atom ratio may be between 0.1/99.9 to 2.0/98. The solid solution containing FeS2 and copper, for example, is formed by mixing FeS2 powder with Cu2S (described in the working example) and heat treating the mixture in an atmosphere of nitrogen at 300° C. for 24 hours. There is no report of incorporation of the Cu into the FeS2 crystalline structure or any changes to the FeS2 crystalline structure or the nature of such changes. The lithium battery with the FeS2 and Cu or Zn added tended to show better performance than same cell with only FeS2 as cathode active material.
The electrolyte used in a primary Li/FeS2 cells are formed of a “lithium salt” dissolved in an “organic solvent”. Representative lithium salts which may be used in electrolytes for Li/FeS2 primary cells are referenced in U.S. Pat. No. 5,290,414 and U.S. Pat. No. 6,849,360 B2 and include such salts as: Lithium trifluoromethanesulfonate, LiCF3SO3 (LiTFS); lithium bistrifluoromethylsulfonyl imide, Li(CF3SO2)2N (LiTFSI); lithium iodide, LiI; lithium bromide, LiBr; lithium tetrafluoroborate, LiBF4; lithium hexafluorophosphate, LiPF6; lithium hexafluoroarsenate, LiAsF6; Li(CF3SO2)3C, and various mixtures. In the art of Li/FeS2 electrochemistry lithium salts are not always interchangeable as specific salts work best with specific electrolyte solvent mixtures.
In U.S. Pat. No. 5,290,414 (Marple) is reported use of a beneficial electrolyte for FeS2 cells, wherein the electrolyte comprises a lithium salt dissolved in a solvent comprising 1,3-dioxolane (DX) in admixture with a second solvent which is an acyclic (non cyclic) ether based solvent. The acyclic (non cyclic) ether based solvent as referenced may be dimethoxyethane (DME), ethyl glyme, diglyme and triglyme, with the preferred being 1,2-dimethoxyethane (DME). As given in the example the dioxolane and 1,2-dimethoxyethane (DME) are present in the electrolyte in substantial amount, i.e., 50 vol % 1,3-dioxolane (DX) and 40 vol % dimethoxyethane (DME) or 25 vol % 1,3-dioxolane (DX) and 75 vol. % dimethoxyethane (DME) (col. 7, lines 47-54). A specific lithium salt ionizable in such solvent mixture(s), as given in the example, is lithium trifluoromethane sulfonate, LiCF3SO3. Another lithium salt, namely lithium bistrifluoromethylsulfonyl imide, Li(CF3SO2)2N is also mentioned at col. 7, line 18-19. The reference teaches that a third solvent may optionally be added selected from 3,5-dimethylisoxazole (DMI), 3-methyl-2-oxazolidone, propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC), tetrahydrofuran (THF), diethyl carbonate (DEC), ethylene glycol sulfite (EGS), dioxane, dimethyl sulfate (DMS), and sulfolane (claim 19), with the preferred being 3,5-dimethylisoxazole.
In U.S. Pat. No. 6,218,054 (Webber) is disclosed an electrolyte solvent system wherein dioxolane-based solvent and dimethoxyethane-based solvent are present in a weight ratio of about 1:3 (1 part by weight dioxolane to 3 parts by weight dimethoxyethane).
In U.S. Pat. No. 6,849,360 B2 (Marple) is disclosed an electrolyte for an Li/FeS2 cell, wherein the electrolyte comprises the salt lithium iodide dissolved in the organic solvent mixture comprising 1,3-dioxolane (DX), 1,2-dimethoxyethane (DME), and small amount of 3,5dimethylisoxazole (DMI). (col. 6, lines 44-48.)
In US 2007/0202409 A1 (Yamakawa) it is stated with reference to the electrolyte solvent for the Li/FeS2 cell at para. 33: “Examples of the organic solvent include propylene carbonate, ethylene carbonate, 1,2-dimethoxy ethane, γ-butyrolactone, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, sulfolane, acetonitrile, dimethyl carbonate, and dipropyl carbonate, and any one of them or two or more of them can be used independently, or in a form of a mixed solvent.” Such statement is misleading, since the art teaches only specific combinations of electrolyte solvents will be workable for the Li/FeS2 cell depending on the particular lithium salt to be dissolved in the solvent. (See, e.g. above U.S. Pat. No. 5,290,414 and U.S. Pat. No. 6,849,360) The reference Yamakawa does not teach which combination of solvents from the above list are to be used with any given lithium salt.
In U.S. 2006/0046152 (Webber) is disclosed an electrolyte system for a lithium cell which may have a cathode comprising FeS2 and FeS therein. The disclosed electrolyte contains lithium iodide salt dissolved in a solvent system comprising a mixture of 1,2-dimethoxypropane and 1,2-dimethoxyethane.
The choice of a particular organic solvent or mixture of different organic solvents for use in conjunction with any one or more lithium salts to produce a suitable electrolyte for the Li/FeS2 cell is challenging. This is not to say that many combinations of lithium salts and organic solvents do not produce a Li/FeS2 cell which will not work at all. But rather the challenge associated with such cells using an electrolyte formed with just any combination of known lithium salt and organic solvent is that the problems encountered will likely be very substantial, thus making the cell impractical for commercial usage. The history of development of lithium cells in general, whether lithium primary cells, e.g. non rechargeable Li/MnO2 or Li/FeS2 cells or rechargeable lithium or lithium ion cells reveals that just any combination of lithium salt and organic solvent cannot be expected to result in a good cell, that is, exhibiting good, reliable performance. Thus, references which merely provide long lists of possible organic solvents for Li/FeS2 cells do not necessarily teach combinations of solvents or combination of specific lithium salts in specific solvent mixtures, which exhibit particular or unexpected benefit.
It is desired to produce a primary (nonrechargeable) lithium battery that is reliable and has good rate capability for use under indoor and outdoor conditions.
It is desired to produce a primary (nonrechargeable) lithium cell having good rate capability that the cell may be used in place of rechargeable batteries to power digital cameras.